We previously reported that human mast cells arise from CD34positive human stem cells in SCF (KIT positive). To further characterize the human mast cell precursor, we sorted CD34positive cells using a variety of candidate markers. The most promising was CD13. A series of experiments based on cell sorting and colony formation then led to the conclusion that CD34 positive, KIT positive, CD13 positive cells gave rise to all mast cells in culture. The regulation of mast cell growth and differentiation depends upon the sequential availability of specific growth factors and the expression of their corresponding receptors. We now have shown that alpha-melanocyte stimulating hormone (alpha-MSH) binds to murine mast cells through its receptor, melanocortin- 1(MC-1). This interaction then increases intracellular CAMP, inhibits IgE-mediated histamine release, and down-modulates mRNAs for IL-1 beta, TNF-alpha and lymphotactin. IL-3 dependent proliferative activity by mast cells was slightly but significantly augmented by alpha-MSH.CD44 is expressed in various isoforms on multiple cell lineages including those of hematopoietic origin and is believed in part to mediate cell adhesion to hyaluronic acid. Using a standard assay, CD34+ derived cultured human mast cells were also demonstrated to adhere to hyaluronic-acid-coated surfaces. Human mast cells were then found by flow cytometry to express CD44S, but not the v5, v6, v7, and v8 isoforms, and to shed CD44S following activation induced by PMA or aggregation of Fcepsilon RI. Thus, human cultured mast cells express and shed CD44S, which appears to mediate the attachment of these cells to hyaluronic acid.